1. Field of the Invention
This invention relates to a carrier supporting mat lapped around an outer peripheral surface of a catalyst carrier and disposed within a metal casing where an exhaust converter contains the catalyst carrier in the metal casing.
2. Description of Related Art
Exhaust converter is generally made by containing a cylindrical catalyst carrier within a metal casing. Such a catalyst carrier is generally made of a ceramic or metal. The catalyst carrier is generally held not in direct contact with the casing, and a supporting material is lapped around an outer periphery of the catalyst carrier and placed between the casing and the catalyst carrier.
High temperature gas from an engine flows into the exhaust converter, and the exhaust gas is purified by passing through the catalyst carrier. It is preferable to set the exhaust gas at a high temperature to maintain the efficiency of the catalyst in a good state. Therefore, the supporting material having good heat insulating property is lapped around the outer periphery of the catalyst carrier and placed between the casing and the catalyst carrier, so that the exhaust converter is structured to be capable of maintaining the temperature of the exhaust gas.
However, it is impossible to completely isolate the heat of the exhaust gas from the casing, and therefore, the converter cannot stop increasing of the temperature of the casing and renders the casing expand due to increased temperature of the casing. Since the thermal expansion coefficient of the casing is larger than the thermal expansion coefficient of the catalyst carrier, there raises a problem that contact portions between the catalyst carrier and the casing become rickety.
For example, in Japanese Unexamined Patent Publication No. Heisei 7-4,234, an art has been disclosed in which a catalyst carrier is supported in cutting two foamed sheets containing ammonium-ion-processed vermiculite inorganic fiber material, and binder with a thickness of 4.9 mm into rectangular parts having grooves and tongue pieces. With this art, the foamed sheets can be kept away from receiving tears and wrinkles.
In Japanese Unexamined Patent Publication No. Heisei 3-3,915, disclosed is a foamed antiflaming attachment pad made of a foamed antiflaming sheet material of at least two layers adhered to each other and offset in a prescribed amount in a longitudinal direction. With this technology, no tear or crush occurs when the pad is lapped around a fragile structure.
To hold the catalyst carrier in following the expansion and contraction of the metal casing due to heats, a thermally expandable mat is used in any method. Particularly, the recent trend is to use the catalyst carrier at a higher temperature.
A conventional expandable mat including vermiculite cannot be said as having high heat resistance and can be easily impaired. A special processing is therefore required, resulting in that the mat becomes expensive. In addition, the catalyst carrier may be broken due to uneven exertion of areal pressure when the catalyst carrier lapped with such a mat is assembled to the metal casing. Particularly, because the expansion mat including the vermiculite is hard, there raises a problem that binding load may not be so large to hold directly the catalyst carrier having fine cells.
As shown in FIG. 5, a supporting member 51 is formed with a thickness corresponding to a diameter of a catalyst carrier 52 and a size of a casing not shown. When the supporting member 51 is lapped around the outer periphery of the catalyst carrier 52, wrinkles 51a may occur on a contact surface of the catalyst carrier 52 from peripheral length differences between the surface on a side of the catalyst carrier 52 and the surface on an outer side, and also tears 51b may occur on the outer surface. Where such wrinkles 51a and tears 51b occur, there raises a problem to cause breakdown due to uneven exertion of loads to the catalyst carrier 52 when contained in the casing.
The technologies disclosed in the respective Patent Publications cannot solve the above problems. That is, though parts of the respective mats are adhered to create sliding between the respective layers when the mats are lapped around the catalyst carrier, the respective mats have large undulations, so that it is difficult to slide the mats having large friction coefficient.
As described above, where the mats formed in a multilayer form are directly in contact with the catalyst carrier, there raises a problem that the respective portions are not easily separated from each other when the catalyst carrier, the mats, and the like are required to be recycled.
It is an object of the invention to provide a carrier supporting mat for exhaust converter having high heat isolation property and good heat expansion characteristics, not subjecting to wrinkles or tears when lapped around a catalyst carrier.
To accomplish the above problem, according to the invention, a carrier supporting mat for exhaust converter, which includes a metal casing and a catalyst carrier disposed within the metal casing, to position the catalyst carrier in the metal casing upon lapped around the catalyst carrier, includes a multilayer structure having: a highly heat insulating mat member disposed on an inner side; and an expansible mat member containing a vermiculite material disposed on an outer side, and at least one sheet member placed between the respective mat members.
With the carrier supporting mat for exhaust converter, the expansible mat member containing the vermiculite is expanded and contracted according to changes of the temperature of the expansible mat member. Therefore, when the exhaust gas heated at a high temperature flows into the exhaust converter, the highly heat insulating mat member disposed on the inner side insulates the catalyst carrier from the heat. With respect to the thermal expansion of the metal casing, a gap formed between the metal casing and the catalyst carrier can be stuffed by expansion of the expansible mat member containing the vermiculite material disposed on the outer side.
Placement of the sheet member between the respective mat members particularly realizes good sliding among the mat members when the carrier supporting mat is overlapped around the catalyst carrier, thereby allowing the respective mat members to move easily. This can therefore prevent the carrier supporting mat from subjecting to wrinkles or tears. Placement of the mat members also enforces the carrier supporting mat otherwise relatively fragile and easily torn, so that the carrier supporting mat can be overlapped on the catalyst carrier with larger winding force, and so that the highly heat insulating mat member and the expansible mat member, which constitute the carrier supporting mat, can be made thinner.
In the carrier supporting mat, the sheet member is preferably made of a heat resistance sheet. Where the sheet member has a heat resistance property, the respective mat members remain separated by the sheet member as a boundary even though used for a long period, and therefore, when required to be recycled, the catalyst carrier and the highly heat insulating mat member and the expansible mat member, which constitute the carrier supporting mat, can be separated easily.
With the carrier supporting mat, it is preferable to set the thickness of the highly heat insulating mat member disposed on the inner side is made thicker than the thickness of the expansible mat member containing the vermiculite material disposed on the outer side. The carrier supporting mat thus constructed can make the thicknesses of the respective mat members thinner than those of the conventional supporting materials. Therefore, when overlapped around the outer periphery of the catalyst carrier, no wrinkle or tear occurs because the respective mat members have thinner thicknesses.
With the carrier supporting mat, it is preferable to set the length of the highly heat insulating mat member disposed on the inner side is made shorter than the length of the expansible mat member containing the vermiculite material disposed on the outer side, and also, it is preferable to set the highly heat insulating mat member disposed on the inner side and the expansible mat member containing the vermiculite material disposed on the outer side are positionally shifted to one another in an outer peripheral surface direction and adhered at the overlapped portion to one another by the sheet member.
The carrier supporting mat thus structured can be lapped on the catalyst carrier without exposing the outer periphery of the catalyst carrier. A stable lapping state can be realized without positionally shifting of the respective mat members when lapped around the catalyst carrier by adhering the overlapped portions of the highly heat insulating mat member and the expansible mat member to each other by the sheet member. The mat members readily slide along the sheet member disposed between the mat members with respect to the adhered portion as an original point and move relatively, so that assembling efficiency can be improved where no mat member moves independently. Such adhering means losses griping force when heated at a high temperature, so that the carrier supporting mat can be recycled without any adverse effect from the adhered structure.
With the carrier supporting mat, it is preferable to set the highly heat insulating mat member disposed on the inner side and the expansible mat member disposed on the outer side are adhered at an end of the respective mat members in a longitudinal direction of the mat members. The carrier supporting mat thus structured can lap the highly heat insulating mat almost completely around the outer periphery of the carrier.